1. Field of the Invention
The present invention is directed to the purification of antifreeze and, in one aspect, to the removal of heavy metals and arsenic from antifreeze and the disposal of by-products in an environmentally acceptable manner.
2. Description of Related Art
Arsenic and its compounds are used in industry as pesticides, insecticides, and corrosion inhibitors. Many uses of arsenic have been discontinued because of environmental concerns about toxicity to both animals and humans. Nevertheless arsenic compounds are widely scattered throughout the environment. Continuing sources of arsenic atmospheric contamination are coal burning and copper smelting. The Environmental Protection Agency lists arsenic as a carcinogen. Federal and state regulations impose strict limits on arsenic concentrations in soil, air, and water. For example, the Toxic Substance Control Act (TSCA) has a reportable spill quantity of one pound for arsenic.
The effort to remove arsenic compounds from the environment includes efforts to remove it from antifreeze that protects the radiators of engines used with large compressors in the natural gas processing and pipeline industry. Antifreeze is a solution of ethylene glycol and water, the amount of ethylene glycol depending on the lowest anticipated winter temperature, typically in the range of forty to sixty percent by volume. Since arsenic compounds have been used as corrosion inhibitors, many such industrial cooling systems are contaminated with arsenic.
If new antifreeze and new corrosion inhibitors are used, old solutions with arsenic in them must be disposed of in an environmentally safe manner. Burning arsenic contaminated antifreeze may increase air pollution in the same manner as burning coal with arsenic contaminated ash. The continued use of contaminated antifreeze poses a human health risk problem in the event of spills and worker exposure. An antifreeze spill of 1200 gallons containing 100 mg per liter arsenic is a reportable spill. The reportable spill quantity for ethylene glycol is 5000 pounds or 9000 gallons as antifreeze. The arsenic content of contaminated antifreeze may range to 500 mg per liter. The level of heavy metals may be up to 10 ppm. Volumes of contaminated antifreeze may range up to over 50,000 gallons at a single location.
Certain prior art processes are used to remove the build-up of minerals and metals (e.g. barium, cadmium, lead, chromium, copper) from used antifreeze solutions. The general processes involve use reverse osmosis technology; ultra-filtration technology; treatment with molecular sieves; treatment with ion exchange resins; and treatment with activated carbon. These processes may remove arsenic at low concentrations (e.g. at 10 to 20 ppm) but the treatment becomes uneconomical at high arsenic concentrations (e.g. at 25 ppm) because of the low efficiency and poor specificity. Certain prior art chemical treatment processes remove arsenic from antifreeze. One of these employs polyacrylate (a co-polymer of acrylic acid and acrylamide) and ethylene diamine tetra-acetic acid (EDTA) as its major components. Another process uses a formulation with sodium nitrite and potassium hydroxide as its major components. In one prior art process iron sulfate or alum is used to remove arsenic from wastewater. In certain nonanalogous prior art methods heavy metals are removed from water by increasing pH by adding a base (e.g. calcium hydroxide or magnesium hydroxide) until the metals precipitate.